1. Field of the Invention
The present invention relates to a gas turbine combustor which stably burns a flame-retardant gas that has a high content of nitrogen (N2), carbon dioxide (CO2) or water vapor, such as a blast furnace gas, a coal gasified gas or a biomass gasified gas, and is a low calorific gas (low BTU gas).
2. Description of Related Art
In general, a low BTU gas has a low flame temperature and a low burning velocity when compared with an LNG (liquefied natural gas), which is a main fuel of the gas turbine, so the low calorific gas is a fuel that is hard to burn. However, its low NOx emissions during combustion are one of features, so a method of using a low BTU gas is an issue.
A typical example of this low BTU gas is a blast furnace gas. The blast furnace gas is a by-product gas generated from a blast furnace in a steel production process. Recently, there has been an increasing need to use this gas as a gas turbine fuel. The blast furnace gas is a flame-retardant gas that includes carbon monoxide (CO) and hydrogen (H2) as the main flammable components and also includes a large amount of N2 or CO2.
Therefore, it is difficult to use mono-fuel combustion of a blast furnace gas to continue an operation from gas turbine ignition to a full load range. To stably continue an operation (combustion) from ignition to a partial load range in which a combustion temperature is low, it is necessary to perform carburetion during operation by mixing a coke oven gas including hydrogen or an LNG or liquefied petroleum gas (LPG), which is a high BTU gas, into a blast furnace gas or to separately provide a high calorific fuel path used for startup. Since a flame-retardant gas needs to be stably burned, it is a general practice for a gas turbine combustor to use diffusion combustion, in which a fuel and air are supplied through separate flow paths.
Other low BTU gases include gasified gases of coal and biomass. From the viewpoint of using resources efficiently, there has been also an increasing need to use a fuel made from coal or biomass as a gas turbine fuel. A fuel that is obtained by using coal, wood chips, or the like as a raw material and using air for gasification is a law BTU gas that includes a large amount of N2. As in the case of a blast furnace gas, therefore, such a gas needs to be used together with a fuel used for startup and a burner that can burn a low BTU gas is required.
As described above, in general, a low BTU gas has a low flame temperature and a low burning velocity when compared with high calorific fuels such as an LNG, so the low BTU gas is a fuel that is hard to burn. As for the gas turbine combustor, therefore, a technology that stably burns a low BTU gas is an important issue.
Due to a lower heating value, to obtain a combustor exit gas temperature equivalent to that of a high BTU gas such as an LNG, it is necessary to increase the volumetric fuel flow of a fuel supplied to the combustor. Thus, one feature of a low BTU gas fired combustor is that the volumetric fuel flow of a fuel to be supplied is increased.
An example of the structure of a low BTU gas fired burner is described in the Patent Literature 1. In the Patent Literature 1, a structure is used in which a liquid fuel nozzle for startup is provided at the center in radial directions of a burner, gas holes are formed along an outer circumference of the liquid fuel nozzle, and a gas hole and an air hole are further formed alternately along an outer circumference of the outer circumference of the liquid fuel nozzle. This burner is targeted at a low BTU gas including a large amount of N2 such as a coal gasified gas.
With a burner that holds a flame with a swiveled jet, to hold a flame, it is generally necessary to form a recirculation zone in the vicinity of the center in radial directions of the burner and give thermal energy to air and a fuel jetted from the burner.
In the Patent Literature 2, a low BTU gas is aggressively used to form a recirculation zone. It is characterized in that gas holes are formed in an inner swirler and most of a fuel is supplied to the inner swirler so that a strong swiveled flow is formed by using the momentum of a large amount of low BTU gas and flame holding is enhanced. The fuel jetted from the inner swirler is supplied to the recirculation zone while being mixed with air jetted from an outer swirler, so oxygen in the zone does not become insufficient and stable combustion of a low BTU gas is possible.
{Patent Literature}
{Patent Literature 1}
Japanese Patent Laid-open No. Hei 5 (1993)-86902
{Patent Literature 2}
Japanese Patent Laid-open No. 2005-241178